1. Field of the Invention
This invention relates to a method for preparing monosilanes from a high-boiling fraction of methylchlorodisilanes formed as by-products in the direct synthesis of methylchlorosilanes.
2. Prior Art
It is well known to produce methylchlorosilanes by reacting metallic silicon and methyl chloride as reactants in the presence of copper catalysts. There are produced monosilanes such as dimethyldichlorosilane, trimethylchlorosilane, methyltrichlorosilane, and methylhydrogen-dichlorosilane. This process is often referred to as the direct synthesis of methylchlorosilanes. The direct synthesis process is disclosed in Rochow, U.S. Pat. Nos. 2,380,995 and 2,488,487. It is a predominant industrial process for the manufacture of monosilanes.
A variety of modified silanes, silicon-containing compounds and silicone polymers are derived from these monosilanes utilizing their reactive sites. These derivatives now find use in electric, cosmetic and other industries depending on their properties.
The direct synthesis of methylchlorosilanes yields not only the desired monosilanes mentioned above, but also a high-boiling fraction of methylchlorodisilanes of the general formula (1): EQU (CH.sub.3).sub.a Si.sub.2 Cl.sub.6-a ( 1)
wherein letter a is an integer of 2 to 6.
In the industrial scale synthesis of methylchlorosilanes in the state of the art, this high-boiling fraction usually accounts for about 10% of a crude product. It is economically and ecologically important to convert the high-boiling fraction into valuable monosilanes for reducing waste residues.
One well-known method for converting the high-boiling fraction into monosilanes is by reacting the high-boiling fraction with hydrogen chloride. The method for converting the high-boiling fraction into monosilanes through reaction with hydrogen chloride is disclosed in JP-B 6869/1957, 8458/1968, French Patent No. 1119915, W. Noll, Chemie und Technologie der Silicone, 2nd Ed., 1968, Chap. 2.2.8, and Pachaly, U.S. Pat. No. 5,288,892. These publications describe:
(i) that methylchlorodisilanes are converted into monosilanes by cleaving Si--Si bonds in methylchlorodisilanes using a tertiary amine and halogenic acid; PA1 (ii) that reaction is carried out in an evaporator with an attached column; PA1 (iii) that by-products are separated into disilanes cleavable with hydrogen chloride and noncleavable disilanes, and the cleavable disilanes are subject to reaction; and PA1 (iv) that this separation is continuously carried out.
In these prior art methods, however, monosilanes are recovered in low yields because the conversion of high-boiling residues into monosilanes is insufficient.
After the high-boiling fraction is reacted with hydrogen chloride, there is left unreacted hydrogen chloride. One common treatment of the unreacted hydrogen chloride is scrubbing with water. This treatment has the drawback that the effective utilization of scrubbed hydrogen chloride generally requires a large size of equipment and enormous energy consumption. The scrubbing treatment is also economically disadvantageous in that it adds to the load for the disposal of plant waste water containing hydrogen chloride.